electromagnetic fields, the receiving antenna must be located in the plane of polarization. This places the conductor of the antenna at right angles to the magnetic lines of force moving through the antenna and parallel to the electric lines, causing maximum induction. ">

For maximum absorption of energy from the electromagnetic fields, the receiving antenna
must be located in the plane of polarization. This places the conductor of the
antenna at right angles to the magnetic lines of force moving through the antenna and
parallel to the electric lines, causing maximum induction.

Normally, the plane of polarization of a radio wave is the plane in which the E field
propagates with respect to the Earth. If the E field component of the radiated wave
travels in a plane perpendicular to the Earth's surface (vertical), the radiation is said
to be VERTICALLY POLARIZED, as shown in figure 2-5, view A. If the E field propagates in a
plane parallel to the Earth's surface (horizontal), the radiation is said to be
HORIZONTALLY POLARIZED, as shown in view B.

Figure 2-5. - Vertical and horizontal polarization.

The position of the antenna in space is important because it affects the polarization
of the electromagnetic wave. When the transmitting antenna is close to the ground,
vertically polarized waves cause a greater signal strength along the Earth's surface. On
the other hand, antennas high above the ground should be horizontally polarized to get the
greatest possible signal strength to the Earth's surface. Vertically and horizontally
polarized antennas will be discussed in more detail in chapter 4.

The radiated energy from an antenna is in the form of an expanding sphere. Any small
section of this sphere is perpendicular to the direction the energy travels and is called
a WAVEFRONT. All energy on a wavefront is in phase. Usually all points on the wavefront
are at equal distances from the antenna. The farther the wavefront is from the antenna,
the less spherical the wave appears. At a considerable distance the wavefront can be
considered as a plane surface at a right angle to the direction of propagation.

If you know the directions of the E and H components, you can use the "right-hand
rule" (see figure 2-6) to determine the direction of wave propagation . This rule
states that if the thumb, forefinger, and middle finger of the right hand are extended so
they are mutually perpendicular, the middle finger will point in the direction of wave
propagation if the thumb points in the direction of the E field and the forefinger points
in the direction of the H field. Since both the E and H fields reverse directions
simultaneously, propagation of a particular wavefront is always in the same direction
(away from the antenna).

Figure 2-6. - Right-hand rule for propagation.

Q.8 If a transmitting antenna is placed close to the ground, how should the antenna be
polarized to give the greatest signal strength?
Q.9 In the right-hand rule for propagation, the thumb points in the direction of the E
field and the forefinger points in the direction of the H field. In what direction does
the middle finger point?

Within the atmosphere, radio waves can be reflected, refracted, and diffracted like
light and heat waves.

Reflection

Radio waves may be reflected from various substances or objects they meet during travel
between the transmitting and receiving sites. The amount of reflection depends on the
reflecting material. Smooth metal surfaces of good electrical conductivity are efficient
reflectors of radio waves. The surface of the Earth itself is a fairly good reflector. The
radio wave is not reflected from a single point on the reflector but rather from an area
on its surface. The size of the area required for reflection to take place depends on the
wavelength of the radio wave and the angle at which the wave strikes the reflecting
substance.

When radio waves are reflected from flat surfaces, a phase shift in the alternations of
the wave occurs. Figure 2-7 shows two radio waves being reflected from the Earth's
surface. Notice that the positive and negative alternations of radio waves (A) and (B) are
in phase with each other in their paths toward the Earth's surface. After reflection takes
place, however, the waves are approximately 180 degrees out of phase from their initial
relationship. The amount of phase shift that occurs is not constant. It depends on the
polarization of the wave and the angle at which the wave strikes the reflecting surface.
Radio waves that keep their phase relationships after reflection normally produce a
stronger signal at the receiving site. Those that are received out of phase produce a weak
or fading signal. The shifting in the phase relationships of reflected radio waves is one
of the major reasons for fading. Fading will be discussed in more detail later in this
chapter.

Figure 2-7. - Phase shift of reflected radio waves.

Refraction

Another phenomenon common to most radio waves is the bending of the waves as they move
from one medium into another in which the velocity of propagation is different. This
bending of the waves is called refraction. For example, suppose you are driving down a
smoothly paved road at a constant speed and suddenly one wheel goes off onto the soft
shoulder. The car tends to veer off to one side. The change of medium, from hard surface
to soft shoulder, causes a change in speed or velocity. The tendency is for the car to
change direction. This same principle applies to radio waves as changes occur in the
medium through which they are passing. As an example, the radio wave shown in figure 2-8
is traveling through the Earth's atmosphere at a constant speed. As the wave enters the
dense layer of electrically charged ions, the part of the wave that enters the new medium
first travels faster than the parts of the wave that have not yet entered the new medium.
This abrupt increase in velocity of the upper part of the wave causes the wave to bend
back toward the Earth. This bending, or change of direction, is always toward the medium
that has the lower velocity of propagation.

Radio waves passing through the atmosphere are affected by certain factors, such as
temperature, pressure, humidity, and density. These factors can cause the radio waves to
be refracted. This effect will be discussed in greater detail later in this chapter.

Diffraction

A radio wave that meets an obstacle has a natural tendency to bend around the obstacle
as illustrated in figure 2-9. The bending, called diffraction, results in a change of
direction of part of the wave energy from the normal line-of-sight path. This change makes
it possible to receive energy around the edges of an obstacle as shown in view A or at
some distances below the highest point of an obstruction, as shown in view B. Although
diffracted rf energy usually is weak, it can still be detected by a suitable receiver. The
principal effect of diffraction extends the radio range beyond the visible horizon. In
certain cases, by using high power and very low frequencies, radio waves can be made to
encircle the Earth by diffraction.

Figure 2-9. - Diffraction around an object.

Q.10 What is one of the major reasons for the fading of radio waves which have been
reflected from a surface?